A kinetic reaction mechanism for ammonia– hydrogen flames and its application in aircraft propulsion systems

Alnasif, Ali 2025. A kinetic reaction mechanism for ammonia– hydrogen flames and its application in aircraft propulsion systems. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The increasing risks associated with global warming, primarily driven by greenhouse gas emissions from fossil fuel combustion, have underscored the urgent need for carbon-free energy solutions. Ammonia has emerged as a promising alternative due to its established infrastructure, high hydrogen density, and potential for Net Zero emissions. However, its low burning velocity, high ignition temperature, and NOₓ emissions present significant challenges for practical implementation. Hydrogen enrichment has been shown to enhance ammonia’s combustion characteristics, improving its viability for gas turbines and internal combustion engines. This study investigates the combustion kinetics of NH₃/H₂ flames, aiming to develop and optimise a compact kinetic reaction mechanism that improves predictive accuracy for modelling swirl flame behaviour while achieving low computational time under various combustion conditions. To achieve this objective, two key approaches were employed: (i) a comprehensive evaluation of 67 kinetic reaction mechanisms from the literature to assess their predictive performance across various combustion conditions, alongside an in-depth investigation of NH₃/H₂ flame chemistry using sensitivity analysis, reaction pathway evaluation, and rate-of-production assessments; and (ii) mechanism optimisation through refinement of key reaction rate parameters to enhance kinetic behaviour while maintaining the original 21 species and 64 reactions from the baseline San Diego 2018 mechanism. The optimisation process was conducted by adjusting Arrhenius rate coefficients using Optima++ and the FOCTOPUS algorithm, minimising the root-mean-square deviation (√E) between model predictions and experimental data while ensuring physical consistency. The optimised mechanism was validated across a wide range of equivalence ratios (0.2–2), pressures (0.5–14.8 atm), and temperatures (295–1281 K), as well as full hydrogen blending ratios (0–100%). Model accuracy was assessed using the normalised RMSD error, where √E ≤ 1 represents an ideal fit, √E ≈ 2 indicates high reliability, and √E < 3 is considered satisfactory for combustion applications. The findings demonstrate that existing well-known kinetic mechanisms struggle to consistently predict the combustion characteristics of the fuel, particularly in terms of laminar burning velocity (LBV) and NOₓ formation. The developed mechanism enhances LBV predictions, achieving a normalised RMSD error of √E = 1.97, demonstrating its high accuracy in capturing LBV trends. In jet-stirred reactor experiments, it attained a normalised RMSD error of √E = 2.72, reinforcing its predictive reliability. Furthermore, in stabilised burner-stagnation flame tests, it ranked second-best among 21 mechanisms, with an error of √E = 3.24, significantly outperforming the baseline model of San Diego 2018 (√E = 13.9). Computationally, the optimised mechanism reduced CFD simulation time by a factor of 2.14 compared to the Stagni (2020) model and 1.78 compared to the Nakamura (2019) mechanism, cutting simulation duration from 16 and 13 hours to 7.5 hours, respectively, on 96 CPU cores, while maintaining accurate emission predictions. These results highlight the optimised mechanism’s ability to balance computational efficiency and predictive accuracy, making it a robust tool for modelling ammonia-hydrogen flames. Its enhanced performance in predicting flame speed and species concentrations provides a validated kinetic framework for CFD simulations, supporting the design and optimisation of ammonia-based combustion systems. By addressing key challenges in prediction accuracy and computational efficiency for swirling flame simulations, this study contributes to the development of ammonia-based combustion technologies for low-emission energy generation. The findings provide a pathway for integrating ammonia into gas turbines and other combustion applications aligned with Net Zero emission strategies, further advancing its role in future clean energy system.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Engineering
Uncontrolled Keywords:	1. Ammonia- Hydrogen combustion 2. Kinetic reaction mechanism optimisation 3. Laminar flame speed prediction and validation 4. NO and N₂O chemistry prediction and validation 5. NOx chemistry analysis 6. Arrhenius parameter tuning
Date of First Compliant Deposit:	26 November 2025
Last Modified:	26 Nov 2025 15:25
URI:	https://orca.cardiff.ac.uk/id/eprint/182680

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